1. Field of the Invention
The present invention relates to a motor drive circuit.
2. Description of the Related Art
In electronic equipment such as a notebook computer, a fan motor is used for cooling a heat-producing component such as a processor. When driving the fan motor, there may be employed a motor drive circuit that gradually changes a driving voltage of a motor coil in order to reduce noise in the fan motor (See Japanese Patent Laid-Open Publication No. 2004-166379, for example.)
FIG. 4 is a diagram illustrating an example of a configuration of a motor drive circuit for driving a single-phase fan motor. A motor drive circuit 100 controls driving voltages VOUT1 and VOUT2 for driving a motor coil L according to hall signals VH1 and VH2 opposite in phases to each other, which signals indicate a rotational position of a rotor in the motor and are output from a hall element 210. The driving voltages VOUT1 and VOUT2 are voltages to be respectively applied to terminals OUT1 and OUT2 to which the motor coil L is connected. A power supply circuit (REG) 200 generates a predetermined voltage to be supplied to the hall element 210. The hall element 210 outputs the sinusoidal hall signals VH1 and VH2 at a predetermined level having a frequency corresponding to a rotation speed of the fan motor. The hall signal VH1 is input to a resistor 230 connected to an inverting input terminal (hereinafter referred to as negative input terminal) of an operational amplifier 220, while the hall signal VH2 is input to a non-inverting input terminal (hereinafter referred to as positive input terminal) of the operational amplifier 220. Gate voltages of an NMOS transistor 250 and a PMOS transistor 260 for directly driving the motor coil L are changed according to an output of the operational amplifier 220. An inverter 240 is a buffer for driving the NMOS transistor 250 and the PMOS transistor 260. Drain voltages of the NMOS transistor 250 and the PMOS transistor 260 become a driving voltage VOUT1 to be applied to the terminal OUT1. The driving voltage VOUT1 is fed back to the negative input terminal of the operational amplifier 220 through a resistor 270. Therefore, as shown in FIG. 5, the driving voltage VOUT1 is a voltage obtained by amplifying a difference between the hall signals VH1 and VH2 with a gain corresponding to a ratio between the resistor 230 and the resistor 270. Here, the gain determined by the resistors 230 and 270 are set sufficiently great, so that a maximum value of the driving voltage VOUT1 is saturated at a power supply voltage VDD. The driving voltage VOUT2 to be applied to the terminal OUT2 is controlled so as to become opposite in phase to the driving voltage VOUT1 according to the hall signals VH1 and VH2.
As mentioned above, the motor drive circuit 100 gradually changes the driving voltages VOUT1 and VOUT2 according to the hall signals VH1 and VH2, and thus, the noise of the fan motor can be reduced.
In the motor drive circuit 100, it is required to change a level of the power supply voltage VDD in order to control the rotation speed of the fan motor. In more detail, if the power supply voltage VDD is lowered, the maximum voltage of the driving voltage VOUT1 is lowered, and thus, the rotation speed of the fan motor is decreased. On the other hand, if the power supply voltage VDD is raised, the maximum voltage of the driving voltage VOUT 1 is increased, and thus, the rotation speed of the fan motor is increased. If the power supply voltage VDD is lowered in order to decrease the rotation speed of the fan motor, the gain determined by the resistors 230 and 270 is constant, and thus, a proportion of the rise time and fall time is decreased in the driving voltage VOUT1. Therefore, when the power supply voltage VDD is low, the driving voltage VOUT1 gets close to a square wave, and the noise when the fan motor is driving is increased. On the other hand, if the power supply voltage VDD is raised in order to increase the rotation speed of the fan motor, the proportion of the rise time and fall time is increased in the driving voltage VOUT1. When the driving voltage VOUT1 is at a level of an intermediate voltage VDD/2 of the power supply voltage VDD, for example, the NMOS transistor 250 and the PMOS transistor 260 are both turned on. That is, in the driving voltage VOUT1, when the proportion of the rise time and fall time is increased, time gets longer during which the NMOS transistor 250 and the PMOS transistor 260 are both ON, and consumption current of the motor drive circuit 100 is increased. If the proportion of the rise time and fall time in the driving voltage VOUT1 is changed according to change of the power supply voltage VDD as above, problems occur such as increase in noise and increase in consumption current.